Skimping on sleep linked to faster brain aging

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Quiz: Are you a sleep slouch?

European researchers tracked the sleep of more than 5,000 middle-aged folks for five years and found that those who shifted away from regularly sleeping seven to eight hours a night -- to either less sleep or more -- had a faster decline in memory, reasoning skills, and vocabulary than those who maintained good sleep habits.

The researchers acknowledged that the differences in cognitive abilities among the different sets of sleepers were "relatively small" but added that they could be predictive of long-term outcomes like the age when dementia starts. The study was funded by the National Institute on Aging. 

While the study found a link between poor sleep habits and declining mental function, it didn't prove that the first caused the second. Other sleep-altering factors such as depression, chronic diseases, or various medications -- which the study didn't account for -- can also lead to memory loss or mental haziness.

"These are potential explanations of the associations we observed and future work certainly must look at them," says study leader Jane Ferrie, a senior research fellow at the University College London Medical School in Great Britain.

What's not known is why a longer night of sleep could also hinder brain function. It could be, Ferrie speculates, that those who sleep for longer periods have more shallow disrupted sleep instead of the deep restorative kind.

Overall, though, the research adds to accumulating evidence that poor sleep habits are bad for your health. "It affects your mental performance, increases your risk of heart disease and diabetes, and may even shorten your life span," says sleep specialist Dr. Lawrence Epstein, an instructor of medicine at Harvard Medical School.

Your brain depends on sleep to catch up on mental filing and organizational tasks -- cleaning up all the messes made during a busy day of activity. "It's a time for growth, consolidation, memory formation," says Epstein. "It's not just a blank period of time."

Other research has found, for example, that those who took a nap after memorizing a list of words had higher scores on recall tests than those who skipped the nap.

The reasons why some people alter their sleep habits as they enter their 40s and 50s are varied, Epstein says, but they could be experiencing a shift in their circadian clocks to an earlier wake-up time in the morning. That's something that frequently occurs with aging, and if people don't compensate with earlier bedtimes, they could become sleep-deprived.

While most of us should aim for that sweet spot of seven to eight hours of sleep a night, a small minority might need substantially less or substantially more.

How to tell how much sleep you need? Go to bed one night -- preferably at the beginning of the week when you're not sleep-deprived -- and don't set your alarm clock; allow your body to wake up naturally in the morning and see how you feel the rest of the day.

"If you're feeling fully alert and not drowsy, you've probably had enough sleep," says Epstein. "No question, a few people do fine on only five or six hours a night, but those numbers are far fewer than the ones who are actually doing it."